The directive follows a mid-flight incident last week that forced a Southwest flight to execute an emergency landing in Arizona.

"Safety is our number one priority," Transportation Secretary Ray LaHood said in a statement. "Last Friday's incident was very serious and could result in additional action depending on the outcome of the investigation."

Boeing said in a statement that it was cooperating with the FAA and was preparing a service bulletin that will recommend lap-joint inspections on certain 737-series aircraft.

The Southwest Airlines airplane that ruptured last week is an older 737-300 -- a model that is being sent into retirement by U.S. carriers.

Ray Neidl, an independent airline consultant, described the Boeing 737-300 as an "older aircraft mainly phased out in the U.S."

The 737-300s was a much more common sight decades ago, when Boeing (BA, Fortune 500) built and sold more than 1,000 of the airplanes to a variety of carriers around the world. Since then, the 300 model of the 737 has been gradually retired or sold off by the major carriers.

Boeing spokesman Miles Kotay said that 1,113 of the 737-300s were built and sold to various airlines in the 1980s and 1990s, including the sale of 150 of these airplanes to Southwest in the early 1980s.

Boeing no longer makes the 737-300, said Kotay, and doesn't keep track of the planes once they're sold, so he did not know how many of the 737-300s were still in use.

The 737-300 has been in the spotlight since Friday, when the hull of a Southwest (LUV, Fortune 500) plane flying from Phoenix to Sacramento, Calif., ruptured mid-flight, prompting the pilots to make an unscheduled landing at a military base in Yuma, Ariz. The airline said no passengers were injured, except for some pain in the eardrum from a rapid descent, and a flight attendant suffered facial bleeding.

Since then, Southwest has canceled hundreds of flights and grounded scores of planes for inspection, but said potential problems with the hull appear to apply to a limited number of aircraft.

Southwest has a total fleet of 548 airplanes, all of them 737s. But of this total, only 171 of the 737s are the 300 models, said company spokeswoman Christi McNeill.

In a statement on its website, Southwest said three of its 737-300s failed inspection and have been taken out of circulation for repairs. So far, 64 of the planes have passed and been returned to service, according to the airline, leaving 12 more that require inspection.

Southwest will have complied with the FAA directive when all 79 planes have been inspected, the company said.

Passengers were still being affected Monday, when Southwest canceled 70 flights out of a total of 3,400 as a result of the inspections. But the impact was winding down from Sunday, when the airline canceled 300 flights.

Southwest isn't the only U.S.-based airline that's still flying the 300 model.

US Airways (LCC, Fortune 500) spokesman James Olson said that his airline has 19 of the 737-300s, a small part of the company's fleet of more than 600 aircraft. But he said that US Airways' 737-300s are of a "different configuration" than the ones used by Southwest, and are not susceptible to the same type of problem with the hull.

Spokespeople for JetBlue (JBLU), Continental and Alaska Airlines said they do not have 737-300s in their fleets. The online fleet profiles for American Airlines (AMR') and AirTran do not include 737-300s.

United Airlines (UAL) bought 101 of Boeing's 737-300s in the mid-1980s. But a spokesman for United said that the airline retired its 300s in 2008.

Separately, US Airways said Monday that it's inspecting its Airbus A-330-300 jets after discovering a fuel leak in one of the wide-body aircraft. The other eight jets of this model have been taken out of service for inspection, according to US Air spokesman Andrew Christie.